1. Field of the Invention
The present invention relates to, a thin-film transistor for switching display elements and sensor elements, and also to a method for manufacturing a thin-film transistor array (referred to as a "TFT" array hereinafter).
2. Description of the Related Art
In a TFT array to active-matrix-drive a liquid crystal element, as illustrated in FIGS. 8 and 9, there are provided a large quantity of gate buses 2 and of source buses 3 which are intersected with each other. They are made of, e.g., molydenum and formed on a glass substrate 1. The source bus 3 runs over the gate bus 2 at a crossover portion 4 where these gate buses 2 and source buses 3 are intersected with each other. As shown in FIG. 9, an insulating film 5 is employed between the gate bus 2 and source bus 3 not to electrically conduct these two buses 2 and 3 with each other. This insulating film 5, as illustrated in FIG. 9, includes a first insulating film 6 made of silicon nitride, formed on the gate bus 2, and a second insulating film 8 made of amorphous siliconhydride, which is simultaneously formed when a semiconductor film of a TFT 7 is formed on this first insulating film 6. That is to say, this insulating film 5 is a double layer structure. A passivation film 9 made of, e.g., silicon nitride is formed over an entire surface of the substrate 1 including the source bus 3.
As shown in FIG. 8, a gate line 10 and a source line 11 are branched from the respective gate bus 2 and source bus 3 adjacent to the crossover portion 4, and then connected to the corresponding gate electrode and source electrode of TFT 7 which has been formed near the crossover portion 4. This TFT (thin-film transistor) 7 is arranged by a gate insulating film which is made of, e.g., silicon nitride and formed on the gate electrode while the above-described first insulating film 6 is formed; a semiconductor film made of amorphous siliconhydride which is formed on this gate insulating film; a source electrode and a drain electrode which are made of, e.g., aluminium and simultaneously formed on this semiconductor film when the source bus 3 is formed; and a light shield 12 which is made of, e.g., aluminium and covers the semiconductor film over which a passivation film 9 is formed. The drain electrode of this TFT 7 is connected to a pixel electrode 13.
In such a TFT array, shortcircuit defects may unavoidably occur due to various reasons of the TFT array manufacturing process. As these shortcircuit defects, there are shortcircuit problems occurring at the crossover portion 4 between the source bus 3 and gate bus 2, and between the gate electrode and source electrode. Such a shortcircuit defect appears as a line defect, or a point defect when being arranged with a liquid crystal display element, which causes the lower display characteristic of the liquid crystal display element.
As a result, various defect preventing methods have been proposed so as to recover such a shortcircuit defect occurring at a thin-film transistor array, whereby occurrence of the line defect or point defect of the liquid crystal display element can be prevented.
Referring now to FIGS. 8 and 9, one of the conventional method for recovering the shortcircuit defect will be described.
First, under the condition where the passivation film 9 is not yet formed, the insulating inspection, or examination is carried out between one terminal G1 of the gate bus 2 and one terminal S1 of the source bus 3 so as to detect whether or not there is a shortcircuit problem between the gate bus 2 and source bus 3. If the shortcircuit problem is detected, the source bus 3 at a position a between the crossover portion 4 of the source bus 3 and the branching point of the source line 11 is cut out by the laser trimming treatment. As the laser trimming treatment, a YAG laser is normally employed. In the next stage, the insulating inspection is again performed between the terminal G1 of the gate bus 2 and the terminal S1 of the source bus 3. If the gate bus terminal G1 is electrically conducted to the source bus terminal S1, it can be detected that the shortcircuit problem is produced at the crossover portion 4. Consequently, the shortcircuited position of TFT array can be defined. In this case, the position b of the source bus 3, i.e., the portion near the terminal S1 of the crossover portion 4 is cut out by the laser trimming treatment. If the gate bus terminal G1 is not electrically conducted to the source bus terminal S1 during the above-described second insulating inspection, another insulating inspection is subsequently performed between the terminal G1 of the gate bus 2 and the other terminal S2 of the source bus 3. Then, if the gate bus terminal G1 is electrically conducted to the source bus terminal S2, it can be detected that the gate-to-source path of TFT 7 is shortcircuited. As a result, the shortcircuited position can be specified. In this case, the position c of the source bus 3 adjacent to the terminal S2 of the branching point of the source line 11 is cut out by the laser trimming.
As a consequence, the shortcircuit defects occurring at two positions are completely cut out from the source bus 3 located on both sides of these shortcircuited positions. When power is supplied via both terminals S1 and S2 of the source bus 3 to TFT array during the driving operation, TFT 7 is not brought into operation, but other TFTs are operable so that the above-described line defect can be avoided.
According to the above-described conventional method, after the shortcircuit defect has been recovered, the passivation film 9 is formed over the entire surface of the substrate 1, and then, the light shield for TFT 7 is formed. Thereafter, the resultant substrate 1 is transferred to the subsequent liquid crystal injection step.
Since, however, the power must be supplied via both terminals of the source bus to the TFT array so as to drive the liquid crystal display element, the total number of the required terminals is twice as much as the number of the source bus. Consequently, the respective terminals must be juxtaposed, resulting in occurrence of the shortcircuit problems.
Moreover, if there are more than two shortcircuit problems at a single source bus, no power is supplied to TFT located between these shortcircuited positions. If these shortcircuited positions are separated from each other, the point defects will be continued when driving the liquid crystal display element, which then seems to be the line defect.